Fixed #1252: Warn assigning prototype of "native" object

Wanting to warn on assignment to prototype of native objects.

There are limits to this implementation, it would be very easy to assign native prototypes without being warned. Further, the list of native objects is a static array, and there is no option as of yet to ignore or add additional objects to warn on. Finally, mechanisms like _.extend() are not handled. So, if anyone could suggest some better ways to do this, I'm happy to try and build a more robust version.